module: Factor out elf_validity_cache_index_str

Pull out index validation for the symbol string section.

Note that this does not validate the *contents* of the string table,
only shape and presence of the section.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>